Specifies that the entropy dual and gradient should be
computed with a quadratic penalty term on magnitude of the
parameters. This ‘smooths’ the model to account for noise in the
target expectation values or to improve robustness when using
simulation to fit models and when the sampling distribution has
high variance. The smoothing mechanism is described in Chen and
Rosenfeld, ‘A Gaussian prior for smoothing maximum entropy
models’ (1999).